Add the current chain to the version string

Summary:
This will help identifying which variant of the software is run.

Depends on D8289.

Test Plan:

./src/bitcoind -version
./src/qt/bitcoin-qt -version

Launch bitcoin-qt and click the "Help"=>"About Bitcoin ABC" button.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D8293